1. Field of Invention
The present invention relates to a process for manufacturing free-flowing pellets to be used for fiber reinforcement of thermoplastics. More specifically, the present invention relates to a thermoplastic containing finely distributed free-flowing pellets for use as a thermoplastic for manufacturing fiber-reinforced preforms, and preforms of this type.
2. Description of Related Art
Often, use of cellulosic and/or lignocellulosic fibers as reinforcement components in polymeric matrices represents an increasingly utilized alternative for glass fiber reinforcement, as described for example in WO 02/10272 for thermoplastic matrices. Particularly in the automotive industry, components reinforced with hemp fibers, for example, are used for door trim and similar low-load-bearing structures in the car interior, as described for example in DE 100 52 693. Despite significant progress in recent years, the properties of polymers reinforced with natural fibers do not compare with those reinforced with glass fibers. In particular, there are significant shortcomings in the impact strength.
Maintaining the advantages of cellulosic fibers over glass fibers, such as low density (1.5 vs. 2.5 g/cm3), low abrasion of the processing machines, and good recyclability and disposability, cellulosic spun fibers have recently been used as reinforcement components, primarily in thermoplastic matrices (see Weigel, Ganster, Fink, Gassan, Uihlein: Kunststoffe 92, 2002, pp. 95 to 97, WO 03/033227, and WO 03/016011). Besides the pultrusion process described in WO 03/03227, with a subsequent homogenization stage, which is a two-stage process, there is of course interest in a one-stage process for reasons of cost. The difficulty in this case, in strong contrast to glass fibers, is the introduction of sufficient fiber quantities, about 10 to 50% by mass, into the polymer melt. While such introduction must be gentle in the case of fragile glass fibers, the tougher cellulosic spun fibers are not sufficiently comminuted by the kneading and mixing components in the processing machines. Direct introduction at the start of the extrusion path therefore meets with considerable difficulty. Prior comminution of the fibers or the use of staple fibers, on the other hand, is problematical due to the low bulk density of the fiber material.